Problem: Solve for $k$ : $20 = 10 + k$
Explanation: Subtract $10$ from both sides: $ \begin{eqnarray} \\ \hphantom{20 {- 10}} &\hphantom{=}& \hphantom{k} \\ 20 &=& 10 + k \\ \\ {-10} && {-10} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 20 {- 10} &=& k \\ \end{eqnarray} $ Simplify. $ k = 10$